There are many things you can criticize Linux for, but this is not one of them.
.profile, /etc/profile is a mechanism from the bourne shell, so you would go sh(1) as long as its not bash again. Otherwise mksh(1) or dash(1).
/etc/environment is pam_env(8)
And I didn't understand anything you said after and including bourne shell. I don't know if Windows has any documentation for its environment variables program, but I have never needed documentation for it because it's not brain dead. It's a GUI. An interface which is self-documenting.
I wanna see you find the right dialogue windows in the control panel.
Would you know how to enable/disable IPv6 from your head?
https://www.gnu.org/software/bash/manual/bash.html#Bash-Star...
These files are all shell scripts and largely empty before distribution-specific customizations, there isn't much to document there. They are commands that you want to run in startup.
Nevertheless, your criticisms are fair. However, unlike on Windows, Linux is actually working on that. Distributions like NixOS and Fedora Silverblue are starting to take the whole "put everything under control" very seriously.
I see no comparable efforts on Windows.